Abstract
Background:
Obesity is well recognised as a disease process leading to multiple pathological consequences. Prevalence rates have increased to a point where more than one-third of people worldwide have a body mass index (BMI) ≥ 25 kg.m-2 and for many nations overweight and obesity have become the norm. The obesogenic environment has been blamed for the marked increase in obesity rates in recent years. However, despite this dramatic increase a substantial sector of the population has remained lean, seemingly resistant to the obesogenic environment. Therefore, an alternative approach in determining cause, treatment, and prevention of obesity is to study those who appear resistant to the obesogenic environment. Information from this group should be valuable in developing potential strategies to aid those who continually struggle with their weight.
Objectives:
The overall aims of this thesis were:
1. To compare and contrast physiological, metabolic, behavioural and lifestyle characteristics of individuals who maintain a healthy body weight (BW) with relative ease i.e. obesity resistant individuals (ORI) with those who struggle to maintain a healthy BW i.e obesity susceptible individuals (OSI).
2. To compare the compensation capabilities of ORI and OSI in response to additional dietary energy intake.
Design:
To achieve the first overall aim a cross-sectional study called Born to be Lean (B2BL) was conducted with additional data on eating behaviour and sensitivity to fatty acid ingestion collected in two nested cross-sectional studies using ORI and OSI participants from the Ice Tea (IceT) study. The second overall aim was achieved by the IceT study intervention, a randomised, controlled, double-blind, parallel study. For both studies, participants were classified as ORI or OSI based on their responses to pre-tested screening tool.
For the B2BL study, 34 ORI (17 females, 17 males) and 29 OSI (16 females, 13 males) with no history of chronic disease, thyroid disorder, metabolic disease, or eating disorders, and who were not currently smokers, pregnant, lactating or experiencing menopause, were recruited. Body composition was assessed using dual-energy x-ray absorptiometry (DXA). Fingerprick blood samples to measure ghrelin, total peptide YY (PYY), leptin, insulin and glucose along with appetite ratings measured using visual analogue scales (VAS) were collected at baseline and 15, 30, 60, 120 and 180 min following consumption of a standardised meal. Fasting, area under the curve, peak/nadir and time to peak/nadir were compared. Indirect calorimetry was used to measure resting metabolic rate (RMR) and comparison of measured RMR with three published RMR prediction equations was undertaken. Dietary intake was assessed using a four-day weighed diet record (4DDR). The International Physical Activity Questionnaire (IPAQ) and accelerometer data collected over 7 days was used to assess physical activity (PA) and sedentary behaviour. Dietary restraint, disinhibition and hunger were assessed using the Three Factor Eating Questionnaire (TFEQ).
For the IceT study intervention, 63 ORI and 55 OSI were recruited using the same classification procedures and exclusion criteria as the B2BL study, with the addition of phenylketonuria. Thirty ORI and 27 OSI were randomly assigned to consume a 500 ml sugar-sweetened (SS) beverage (∼1000 kJ), while 33 ORI and 28 OSI were randomly assigned to consume a 500 ml artificially-sweetened (AS) beverage (∼25 kJ) daily for 8 weeks. Body composition (DXA), dietary intake (4DDR), PA (pedometer) and blood lipids (venous blood sample) were assessed at baseline and at the end of the 8 week intervention. In addition, two nested cross-sectional studies were undertaken using participants from the IceT study cohort. In the first study restrained eating, emotional eating and external eating were assessed using the Dutch Eating Behaviour Questionnaire (DEBQ) and intuitive eating was assessed using the Intuitive Eating Scale (IES). The second study assessed oral sensitivity to oleic acid (1.4mM) using triplicate triangle tests and fat ranking ability using samples of custard containing 0%, 2%, 6% and 10% canola oil.
Outcomes:
The results of the B2BL study showed significantly lower absolute RMR (P=0.036) and significantly higher RMR relative to BM (P=0.001) in ORI versus OSI. Female OSI had the lowest relative RMR (kJ.kg-1.d-1) compared to all other groups (P≤0.001). The three RMR prediction equations over-predicted RMR to some extent for male ORI and OSI but especially for female OSI. Levels of restrained eating and disinhibition were significantly lower in ORI versus OSI (P<0.001; P=0.005, respectively) while no significant differences were observed for hunger. Some differences were observed in the response to statements in the attitudes to exercise questionnaire with ORI more likely to agree with the statement ‘I am a sporty person’ (P=0.037) and OSI more likely to agree with the statement ‘I exercise to control my weight’ (P=0.002). No significant differences were observed in the majority of measurements relating to fasting and post-prandial hormone concentrations and appetite responses, dietary intake, physical activity and sedentary behaviour.
The results of the IceT study intervention showed a statistically significant increase in BW, waist circumference (WC) and percentage body fat (%BF) among those consuming the SS beverage compared to the AS beverage (P=0.016), but no interaction between ORS category and the intervention for any of these variables indicating that there was no evidence that the two ORS categories responded differently to the intervention. In the first nested cross-sectional study using participants from the IceT study cohort, significantly lower restrained eating and emotional eating was observed in ORI versus OSI (P<0.001; P=0.010, respectively) while scores for total intuitive eating and the three subscales of the IES were significantly higher in ORI versus OSI (all P≤0.003). Results of the second nested cross-sectional study showed the adjusted odds of being hypersensitive to oral fatty acid ingestion were 3.6 times higher in ORI versus OSI (P=0.034), but no significant differences were detected in the ability to rank the fat content of custard samples in the two ORS categories.
Conclusions:
The results from these studies suggest there are some measurable differences in the characteristics of ORI compared to OSI, especially with regard to RMR, eating behaviour and sensitivity to oral fatty acid ingestion. These findings have implications for both clinical practice and for the design of weight control interventions. The lower RMR relative to BW in OSI compared to ORI results in over-prediction of RMR by prediction equations, making it very difficult for these individuals to achieve the weight loss targets from planned energy restriction, especially for female OSI. Obesity resistant individuals exhibit more healthful eating behaviours and appear be more sensitive to oral fatty acid ingestion than OSI. Both of these characteristics are potentially modifiable and could therefore form part of programmes to assist individuals who struggle with weight control. The compensation capabilities of ORI and OSI were similar in response to additional dietary calories from a beverage, suggesting both groups are vulnerable to the insidious effects of SS beverages on weight gain.